Method of forming aperture plate for electron microscope

ABSTRACT

An electron microscope including an electron source, a condenser lens having either a circular aperture for focusing a solid cone of electrons onto a specimen or an annular aperture for focusing a hollow cone of electrons onto the specimen, and an objective elns having an annular objective aperture, for focusing electrons passing through the specimen onto an image plane. The invention also entails a method of making the annular objective aperture using electron imaging, electrolytic deposition and ion etching techniques.

United States Patent 1 Fletcher et al.

[ 1 Nov. 12, 1974 METHOD OF FORMING APERTURE PLATE FOR ELECTRONMICROSCOPE [76] Inventors: James C. Fletcher, Administrator of theNational Aeronautics and Space Administration with respect to aninvention by; Klaus Heinemann,

1 8,.Me h Du,

Sunnyvale, Ca1if. 94087 7 22 Filed: June 28, 1973 211 App]. No.:374,424

- Related US. Application Data [62] Division or Ser. No. 221,670, Jan.28,

abandoned.

[52] US. Cl 156/7, 156/16, 156/18, 250/495 [51] Int. Cl. C231 1/02 [58]Field of Search 117/215, 217, 71,107,

l17/107.2, 119; 204/23; 156/3, 7, 16, 18; 250/495 R, 49.5 A; 313/64, 74,83, 85 X, 86

[56] References Cited UNITED STATES PATENTS 2,536,383 l/l95l Mears etal....., 156/11 X Pajes 156/7 X Mears 156/3 X Primary ExaminerWilliam A.Powell Attorney, Agent, or Firm-Darrell G. Brekke; Armand G. Morin, Sr.;John R. Manning [57] ABSTRACT 5 Claims, 9 Drawing Figures P I nunv 12I974 A 3,847,689

- sum 1 OF 2 ELECTRON SOURCE CONDENSER LENS I2 A CONDENSER APERTUREPLATE I4 OBJECTIVE LENS I8 OBJECTIVE APERTURE PLATE 2o IMAGE PLANE 24ELECTRON SOURCE\ CONDEN E ENS 1 66 5 0 L CONDENSER APERTURE: 54

PLATE O /SPECIMEN 5e OBJECT5H8/E LENS OBJECTIVE APERTURE PLATE 6OIMAGE6LANE\ V PATENTEDIIOY 12 I974 SHEET 2 OF III Fig 4 Fig 5 Fig 6 Fig7 Fig 8 METHOD OF FORMING APERTURE PLATE FOR ELECTRON MICROSCOPE Theinvention described herein was made in the performance of work under aNASA contract and is subject to the provisions of Section 305 of theNational Aeronautics and Space Act of I958, Public Law 850,568 (72 Stat.435; 42 U.S.C. 2457).

This is a division, of application Ser. No. 22l,670 filed Jan. 28, 1972now abandoned.

SUMMARY OF THE INVENTION The present invention relates generally tocorpuscular ray devices and more particularly to an electron microscopehaving an annular objective lens aperture for eliminating chromaticaberation and inactivate spherical aberation, and a method of making theannular objective lens aperture.

Briefly, the electron microscope of the present invention includes anelectron source, a condenser lens having either a circular aperture forforming a solid cone of electrons onto a specimen or an annular aperturefor focusing a hollow cone of electrons onto the specimen, and anobjective lens having an annular objective aperture for focusing theelectrons passing through the specimen onto an image plane. The circularand annular condenser apertures can conveniently be made usingconventional techniques. However, the much smaller annular objectiveaperture cannot conveniently be provided using prior art methods. Thepresentinvention includes a process for making the objective aperturewhich involves electron imaging, electrolytic deposition and ion etchingtechniques.

A primary advantage of the present invention is that the resolution andbrightness of the high performance electron microscope can besubstantially improved.

Other advantages of the present invention will no doubt become apparentto those of ordinary skill in the art after having read the followingdetailed description of the preferred embodiments which are illustratedin the several figures of the drawings.

IN THE DRAWINGS FIG. 1 is a diagram schematically illustrating anelectron microscope having a set of annular apertures in accordance withthe present invention;

FIG. 2 is a partial plan view of an annular condenser aperture of thetype used in the microscope illustrated in FIG. 1;

FIG. 3 is a partial plan view of an annular objective aperture made inaccordance with the present invention;

FIGS. 4-8 sequentially illustrate a method of making an annularobjective aperture in accordance with the present invention;

FIG. 9 is a diagram schematically illustrating an electron microscopehaving a circular condenser aperture and an annular objective aperturein accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT aperture providedtherein. Disposed on the opposite side of the specimen 16 is anobjective lens 18 having an objective aperture plate 20 with an annularaperture 21 provided therein. The beam 22 of electrons developed bysource 10 is focused by condenser lens 12 and passed through the annularaperture 15 to provide a hollow cone of electrons which are focused to apoint on specimen 16. As the electrons pass through specimen 16 they areagain focused by objective lens 18 through the annular objectiveaperture 21 and onto a point in image plane 24.

In using the annular condenser aperture to provide hollow coneillumination, the zero order of diffraction passes the objective lens ina respective annular zone. It can be shown mathematically that anyimaging process using electrons from a particular zone of the objectivelens is not subject to chromatic aberation in a mathematicalapproximation which is far better than required for experimentalrealization, and that all image information can be given in only oneparticular optimal defocus setting.

It can be shown that any space frequency (reciprocal specimen distances)below a maximum determined by the size of the annular condenser aperturecan be transferred with almost ideally even contrast if conical specimenillumination is used in such a system with an annular objective apertureand the illumination and objective aperture cone angles are identical.

In the preferred embodiment, the size of the condenser aperture 15 isusually on the order of 2-3 millimeters in diameter with the width ofthe open ring area being about microns. As illustrated in FIG. 2 of thedrawing, the inner part of the aperture is supported by three barsdividing the open ring into three areas. Such apertures can bemanufactured using conventional techniques, mechanical or otherwise. Thecorresponding annular objective aperture 21 (FIG. 3) however, is usuallyabout 50 times smaller in diameter than the condenser aperture 15 (e.g.,approximately 50 um in diameter and 3 pm in ring width) and is much moredifficult to manufacture. A preferred method of manufacturing anaperture plate having a suitable objective aperture is illustrated instepwise fashion in FIGS. 4-8 and includes the following steps:

1. First, a collodium film 30 of about 500 angstroms (A) thickness isstretched over a copper specimen grid 32 which is supported on aconventional aperture base 34.

2. A metallic layer or film 36 of several hundred A thickness is thenevaporated onto the upper surface of the collodium film 30 as shown inFIG. 4. The metallic film 36 must be continuous in order to beelectrically conductive. If silver is chosen for film 36, a thickness ofapproximately 300 A is adequate.

3. The prepared composite structure including film 36, film 30, and grid32 is thereafter inserted into the regular objective aperture slider ofan electron microscope, such as that illustrated in FIG. 1, having anannular condenser aperture. With the microscope operated in the selectedarea diffraction mode, the back focal plane of the objective lens, wherethe first image of the annular condenser aperture occurs and where theobjective aperture diaphram is to be located, is imaged onto the imageplane 24 (the microscope screen). With the image of the condenseraperture 15 falling upon the metal film 36, as illustrated in FIG. 5,a'contamination layer 38, caused by the decomposition of residual gasmolecules, forms on top of the metal film 36 in the illuminated area. Asufficient exposure of the electron beam is approximately 1 amp sec/cmwith the microscope operating at lkV. Contamination layer 38 provides apermanent image of the condenser aperture and has annular dimensionscorresponding to those required for the future annular objectiveaperture 21. In other words, the area occupied by the contaminationlayer 38 will be open in the final aperture plate. Note that during thisstep, the electron image can still be observed on the image plane 24because the films 30 and 36 are still electron transparent.

4. The composite structure is then taken out of the microscope andsubmerged in an electrolytic solution comprised of 250g CuSO 1,000 ml H0, and 15 ml H SO and a thin layer of metallic film 40 is galvanicallygrown over the exposed surfaces of film 36 as shown in FIG. 6. The metallayer 40 in the preferred embodiment is of copper and has a thickness ofapproximately 10,000 A.

5. The composite structure is next inserted into an ion etching deviceand is bombarded from beneath, i.e., the side opposite metal layer 40,wtih ions from a gas discharge. The, preferrably Argon, ions will firstetch away the collodium film 30, then the first metal film 36, andfinally part of the second metal film 40 together with the contaminationlayer 38. During this etching process, the aperture is observed with alight microscope to determine when an etching depth sufficient to removethe contamination layer 38 has been reached. As the contamination layer38 has been etched away, the etching process is interrupted.

6. Finally, a third metal layer 42 may be evaporated onto the surface ofthe composite structure for stabilization and cleanliness purposes. Inthe preferred embodiment a layer of gold metal of approximately 1,000 Athickness is provided. At this point the aperture plate is complete andmay be inserted into the electron microscope for use.

Since the characteristics of objective aperture plate 20 are uniquelyrelated to a particular condenser aperture plate 14 and objective lens18, it will be appreciated that the aperture 21 must necessarilycorrespond identically to the aperture 15. Moreover, it will be notedthat since the primary determinant of dimensions of the annular aperture21 is the electron beam cross section, the size of the objectiveaperture can be increased or diminished by simply varying the focusingcharacteristics (focal length) of either condenser lens 12 or objectivelens 18. Since the manufacturing process of the present inventioninvolves operative steps which are inherently highly accurate in thedimensional sense, the resultant aperture is highly accurate. Theproduction of apertures is also highly reproducible.

Using the method of the present invention, it is easily possible toprint several contamination images on one aperture film so that amulti-aperture diaphram can be manufactured featuring one or moreannular apertures in each grid opening. Furthermore, by varying the ob--jective lens current, images of various sizes can be imprinted from thesame original condenser aperture pattern.

The resolution limit of very high resolution electron microscopesoperating in phase contrast, which is the common mode of image formationin high resolution operation and medium excelerating voltage (up tol50kV) microscopes, is determined by'chromatic aberations in theobjective lens. This limitation in resolution can be eliminated if onlythose electrons are permitted to take part in the image formation whichhave passed the objective lens in the same zone, that is, thoseelectrons having the same distance from the optical axis. Suchconditions can be achieved by the use of the annular objective aperture.

Annular objective apertures of the type can be used in the electronmicroscope in two basically different modes of operation which aredependent on the kind of specimen illumination. The two modes ofillumination are (l) axial illumination and (2) complimentary hollowconical illumination. Both methods can be used to obtain ultra-highresolution with strongly reduced chromatic aberation, since theresolution is basically effected only by the imaging parts of theoptical system which is unchanged and characterized by the annularobjective aperture in both cases. The difference between the two modesof operation is in (a) image contrast and (b) width of the transferrablespace frequency band.

In accordance with the present invention, two possible combinations ofthese modes are permitted, i.e., l hollow cone illumination annularobjective aperture, and (2) axial illumination annular objectiveaperture which are illustrated in FIGS. 1 and 9 respectively.

If only the annular objective aperture is used, under paraxia]illumination conditions, selected dark zone field microscopy (SZDF) canbe performed having (a) extremely high contrast because images appear ona black (low noise) background, (b) high resolution because theinfluence of chromatic aberation is eliminated, (c) images of a selectedrange of reciprocal space frequencies only, and thus the possibility ofperforming quantitatively an orientation determination of crystal andspecimens, and (d) defocus dependent Bragg reflection image displacementphenomena, useful for quantitative azimuthal orientation determinationof small individual crystallites.

The advantages of using an annular objective aperture in conjunctionwith a normal circular condenser aperture as is done in selected zonedark field microscopy are illustrated in the article Selected Zone DarkField Electron Microscopy, by Klaus Heinemann and Helmut Poppa, AppliedPhysics Letters, pp. Feb. 1, 1972.

In the first mode illustrated in FIG. 1, i.e., hollow conicalillumination-annular objective aperture, hollow cone illumination isapplied in an electron microscope using an annular condenser aperture asshown in FIG. 1. In this case, the zero order of diffraction passes theobjective lens in an annular zone. Characteristic of this method is thatthe annular zone within which the zero order of diffraction passes theobjective lens is identical with the zone selected by the annularobjective aperture 21. Thus, in this mode two complimentary annularaperture diaphrams are used'simu-ltaneously, one in the illumination.system and one in the imaging system.

It can be shown that any space frequency below a maximum determined bythe size of the annular condenser aperture 15 (reciprocal specimendifferences) can be transferred with almost ideally even contrast ifconical specimen illumination is used with the annular objectiveaperture. Since'this is a bright field mode however, there is no gain incontrast when compared to conventional bright field modes of operation.There is, on the other hand, a considerable increase in beam intensityresulting from the fact that the open area of the annular condenseraperture is much larger (approximately two order of magnitude) than theopen area of a conventional comparable disc aperture. Since,accordingly, the microscope can now be operated with a very low beamcurrent, anomalous beam energy broadening effects (Boersch-Effect) canbe avoided. This adds to the earlier mentioned significant decrease ineffective chromatic aberation.

The second mode (axial illumination-annular objective aperture) isillustrated in FIG. 9. The apparatus for implementing this mode includesan electron source 49, a condenser lens 50 and a condenser apertureplate 52 having a circular aperture 54. Disposed on the other side ofthe specimen 56 are the objective lens 58, an objective aperture plate60 having an annular aperture 62 and the image plane 64.

In this mode the zero order of diffraction is blocked off (the criterionfor dark-field microscopy) by the center portion 63 of aperture plate60, and the annular objective aperture 62 selects a special objectivelens zone only for the image formation. This method has been calledselected-zone-dark-field-microscopy. Contrary to the usual modes ofmicroscopy, this is a typical phase contrast dark field method. Theimage is an interference image between two beams which have beendiffracted at the specimen and passed through the objective lens intoazimuthally different locations in the same zone (with the same apertureangle 0 Consequently, a small aperture width restricts the width of thespace frequency band which is transferrable considerably. This may beundesirable in the case where amorphous specimens are being observed,where practically all distances between specimen details occur andshould be resolved. If, however, crystalline specimens are observed,only discrete object distances, the interplanar distances of the orderedatom planes, are necessary and available to be imaged if the diameter ofthe annular objective aperture is selected properly. such images occurwtih remarkable high contrast, as can be expected in dark fieldmicroscopy.

It is possible that interference between two non-symmetricallydiffracted beams, i.e., two beams which have been Bragg diffracted attwo different overlapping sets of lattice planes with the sameseparation but different azimuthal orientation, can occur. Theseinterferences will, for example, in a [110] oriented f.c.c. crystal,result in simultaneous pseudo" images of [200] and [220] lattice planestogether with the ordinary [1 l l] lattice planes, if the annularobjective aperture was designed for [l l l] Bragg diffraction atcrystalline material of such or similar cell dimensions.

The use of an annularobjectiveaperture in the case of axial illuminationis not only valuable for high resolution images of crystal or graphiclattice planes but can be advantageously applied if the crystal orgraphic orientation of small crystallite has to be determined. This canbe done without direct images of the lattice planes of the cells.

Although it is further contemplated that additional modifications of theabove disclosed invention will no doubt become apparent to those ofordinary skill in the art after having read the above description of thepreferred embodiment, it is to be understood, that this description ismade for purposes of illustration only and is in no way intended to belimiting. Accordingly, it is intended that the appended claims beinterpreted as including all modifications which fall within the truespirit and scope of the invention.

What is claimed is: l. A method of forming an objective aperture platefor an electron microscope having an annular aperture, comprising thesteps of:

disposing an annular aperture in the condenser aperture plane to form ahollow cone of electrons; disposing a first metallic layer in theobjective aperture plane of the focused microscope and in the path ofsaid electrons to cause a contamination layer of residual gas moleculesto form on first said metallic layer in the area illuminated by saidelectrons;

disposing a second metallic layer over that portion of the surface ofthe first metallic layer which is not covered by the contaminationlayer; and

etching the composite structure on the side opposite said secondmetallic layer to a depth transcending said first metallic layer andsaid contamination layer.

2. A method as recited in claim 1 and further comprising the step ofevaporating a third metallic layer over said second metallic layer toadd stability to the resultant aperture plate.

3. A method as recited in claim 1 wherein said first metallic layer isof silver and said second metallic layer is of copper galvanically grownover said first metallic layer.

4. A method as recited in claim 1 wherein said first metallic layer hasa thickness of at least 300 A, and said second metallic layer has athickness of at least 5,000 A.

5. A method as recited in claim 1 wherein said etching step isaccomplished by bombarding said first metallic layer and'saidcontamination'layer with'ions.

1. A METHOD OF FORMING AN OBJECTIVE APERATURE PLATE FOR AN ELECTRONMICROSCOPE HAVING AN ANNULAR APERTURE, COMPRISING THE STEPS OF:DISPOSING AN ANNULAR APERTURE IN THE CONDENSER APERTURE PLANE TO FORM AHOLLOW CONE OF ELECTRONS; DIPOSING A FIRST METALLIC LAYER IN THEOBJECTIVE APERTURE PLANE OF THE FOCUSED MICROSCOPE AND IN THE PATH OFSAID ELECTRONS TO CAUSE A CONTAMINATION LAYER IN THE AREA MOLECULES TOFORM ON FIRST SAID METALLIC LAYER IN THE AREA ILLUMINATED BY SAIDELECTRONS; DISPOSING A SECOND METALLIC LAYER OVER THAT PORTION OF THESURFACE OF THE FIRST METALLIC LAYER WHICH IS NOT COVERED BY THECONTAMINATION LAYER; AND ETCHING THE COMPOSITE STRUCTURE ON THE SIDEOPPOSITE SAID SECOND METALLIC LAYER TO A DEPTH TRANSCENDING SAID FIRSTMETALLIC LAYER AND SAID CONTAMINATION LAYER.
 2. A method as recited inclaim 1 and further comprising the step of evaporating a third metalliclayer over said second metallic layer to add stability to the resultantaperture plate.
 3. A method as recited in claim 1 wherein said firstmetallic layer is of silver and said second metallic layer is of coppergalvanically grown over said first metallic layer.
 4. A method asrecited in claim 1 wherein said first metallic layer has a thickness ofat least 300 A, and said second metallic layer has a thickness of atleast 5,000 A.
 5. A method as recited in claim 1 wherein said etchingstep is accomplished by bombarding said firSt metallic layer and saidcontamination layer with ions.